Chapter 5 Bio f4

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CHAPTER 5 : CELL DIVISION

2 kinds of cell division:

1. Mitosis: division of somatic cells

2. Meiosis: creation of new sex cells

Cell Division Vocabulary

Somatic cell a body cell; a cell whose genes will not be passed on to future

generations.

Germ cell - a cell that is destined to become a gamete (egg or sperm); a cell

whose genes can be passed on to future generations

diploid (2N)

a cell with 2 chromosome sets in each of its cells; all body(somatic) cells

represented by the symbol 2N

Found in somatic or body cells (ex. Skin, digestive tract)

Example : Humans 2N = 46

haploid (N)

a cell with 1 chromosome set in each of its cells; all gametes(sperm, eggs)

represented by the symbol N or 1N

Found in gametes or sex cells sperm & egg

Example: Humans N = 23

MITOSIS

Characteristics of Mitosis

A diploid cell will give rise to a diploid cell

Chromosome number remains the same

The DNA remains identically the same

One cell (2N) gives rise to two cells ( 2N)

the division of the nucleus to produc e two d aughter ce l ls ,each contain

same numb er & same kindof chromosome as the parent cell


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Occurs in al l somat ic cel lsexcept gametes.

Somatic cells contain 2 sets of chromos omes, 1 set from female parent, 1

set from male parent diplo id(2n)

Single set of unp airedchromosome

haploid (n)

Each somatic cells produce 2 new diploid cells identical to the parent cell

SIGNIFICANCE OF MITOSIS

For growth, repair & replaces cells that are dead or damaged

A form of asexual reproduction to increase the number of organisms (Amoeba

sp.)

To ensure that the offsprings/new cells are genetically identical to the parent.

Preserves the diploid number of chromosomes

The Cell Cycle


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INTERPHASE (G1, S, G2)

G1 : Growth phase 1

The cell growth by producing proteins & cytoplasmic organelles

S : Synthesis

Synthesis of DNA, chromosomes are duplicated & DNA has replicated to form 2

identical sister chromatids joined together by centromere

G2 : Growth phase 2

Cell growth & cell differentiation occur

M PHASE(Cell Division)

Mitosis : nucleus divides

Cytokinesis : division of cytoplasm

STAGE OF MITOSIS

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Nucleus divides = cytokinesis (cytoplasm divides)


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1. Prophase

Centrioles move apart to opposite poles

The chromosomes coil up, condense & shorten

Two identical chromatids (sister chromatids) appears, attached at centromere

Nuclear membrane breaks down

Nucleolus disappears

Spindle fibres begin to form extend between the centrioles.

2. METAPHASE

The chromosomes move to the cells

equator

The chromosomes line up along the

equator of the cell with the centromeres

attached to the spindle fibres

Each chromatid of the chromosome faces

its own pole

Metaphase ends when the centromeres

divide


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ANAPHASE

TELOPHASE

Cytokinesis

The division of the cytoplasm.

In animal cells, a Cleavage Furrow forms and separates Daughter Cells

In plant cells, a Cell Plate forms and separates Daughter Cells.

The centromere of each chromosome

divides into two

The sister chromatids of each

chromosome separate and move to

opposite poles of the cell

The spindle fibres pull the centromere

toward each pole

In telophase the nucleus actually divides.

The chromosomes are at the poles of the

cell.

The nuclear envelope re-forms around the

two sets of chromosomes


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APPLICATION OF KNOWLEDGE ON MITOSIS

1. IN CLONING

A TECHNIQUE / the process of producing clones or genetically

identical organisms through asexually reproduction

Contain same genetic content & chromosomal number with one

another as well as with the parent organism

To increase the quantity of the product

To improve the quality, to produce new species & to ensure uniformity

in the traits of the plants

Cloning of animals

1. Reproductive cloning

Producing an entire animal that is genetically identical to the parent animal

The entire animal is produced from a single cell by asexual reproduction

through mitosis.

2. Therapeutic cloning

Is a branch of medicine concerned with the treatment of diseases.

Parts of a person skin, heart, liver or even just a few cells are duplicated to

produce a clone.

The clone tissue is used to replace a damaged or diseases tissue without the

risk of tissue rejection.


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Cloning in plant

1. Tissue culture

Plants can be cloned using tissue culture.

A technique in reproduction which involves the transfer of tissues orcells from an organism into a suitable culture medium to produce a

whole new organism (identical to the existing organism)

Tissue culture produces genetically identical clones.

Tissue culture techniques

a) A pieces of tissue, called explants, its taken from a parents plant (e.g. carrot

root or stem tissue,)

b) The pieces of tissues are sterilised with dilute sodium hypochlorite solution toprevent the growth of pathogens (such as bacteria and fungus).


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c) Each sterile tissue piece is placed on to a growth medium (gel containing

nutrients and growth regulators).

d) The tissues cells divided by mitosis to produce a mass of loosely arranged

undifferentiated cells called callus.

e) Callus is stimulated with shoot-stimulating hormone to form multiple shoots.

f) The shoots are separated and each is placed in nutrient medium with root-

stimulating hormone to encourage rooting

g) Once the roots grow, the plantlets (little plants) are planted in sterile compost

to grow.

ADVANTAGES

1. Produced in a short time (increase quantity)

2. The good qualities of the plants/ animals can be selected & maintained in the

clones

3. Increases the rate of production & the quality of the product

4. Ensure the continuity of hereditary traits from parent to the clones

5. Can be carried out any time of the year


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DISADVANTAGES

1. The resistance of the clones towards diseases & pests is the same. 1 infected

with a disease/pests, all the clones will also affected. Lead to the extinction of

the species.

2. Carried out under controlled environment. External environment changes, the

will be destroyed

3. Prevents natural selection

4. No variation

The effects of uncontrolled mitosis in living things

Mutation is the change in the DNA structure of the cell.

This change in the DNA corrupts the coded genetic instructions for mitosis

control.

This leads to uncontrolled mitosis, which is the non-stop division of cells,

producing a mass of new daughter cells, called tumour.

How cancer occur

- Mitosis is a process of cell division that creates two identical daughter cells,

- each carrying a copy of the original cell's DNA.

- Errors in mitosis result in an incorrect DNA copy; the effect of errors on the

health of the organism range from benign to deadly, depending on the amount

and type of errors.

- One potential consequence is cancer; all cancer types are traced back to

harmful mutations multiplied by mitosis.

Causes of Cancer

1. Genetic- some forms of cancer like prostate, colon, breast, skin, ovary are

suspected to be inherited from the parents

2. Carcinogens- these are chemicals which affect genetic activity and cause

cancer, e.g. of carcinogen a diesel exhaust, cigarette smoke, hair dyes, soot,

arsenic, benzene and formaldehyde.

3. Radiation- excess exposure to x-ray, gamma-rays and ultra violet rays lead to

increase cancer risk.

4. Age- some cancers are found primarily in young people (e.g. leukemia), while

some cancers (e.g. colon cancers) are found mostly in older adults.

5. Viruses- some viruses (such as the EB and HIV-1) cause cancer.


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MEIOSIS

Human Chromosomes

Humans have 23 pairs of chromosomes (46 total)

22 pairs of autosomes

1 pair of sexchromosomes

Half of each pair came from oneparent and half came from the otherparent

Meiosis

A division of the nucleus to produce 4 daughter cells each containing half the

chromosome number of the parent nucleus.

A type of cell division - occurs in reproduction organs to produce 4 daughter

cells called gametes.

Meiosis takes place in the gonads(sexual organs)

For humans, these are the ovaries and testes

The process of meiosis produces eggand spermcells

Two gametes come together by fertilization

The haploidsperm and egg join to form a diploid zygote


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Meiosis Phases

Meiosis I Prophase I Metaphase I Anaphase I Telophase I

Meiosis II Prophase II Metaphase II Anaphase II Telophase II


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Interphase

Before Meiosis (just like before Mitosis) the cell must prepare for division:

Cells increase in size

DNA is replicated


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Necessary proteins

and RNAare

synthesized

During this phase, chromosomes are not yet visible.

Meiosis: Prophase I

Crossing over increases genetic variation

Crossing over happened : non-sister chromatids exchange segments of DNA

new combination of genes on a chromosome

Chiasmata

the points at which segments of chromatids cross over

Chromosomes become visible

Nuclear envelope disappears

Centrioleshead to opposite poles and spindle forms

Homologous chromosomes (one pair of sister chromatids

from the motherand one from the father) pair up to form

a tetrad

The tetrad pairs up so tightly that crossing overoccurs

Crossing over happens when parts of the homologues

chromosomes switch places after overlapping


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METAPHASE I

The paired chromosomes are lined up at the equator of the cell

One chromosome of each pair faces each pole of the cell

the chromosomes attached to the spindle fibres at their centromere

The centromere does not divide

ANAPHASE I

The paired chromosomes separate from one another & move to opposite

poles

The spindle fibres pulled one chromosome of each pair to each pole


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TELOPHASE I

The chromosomes arrive at the poles.

Each pole has a haploid daughter nucleus (contain one set)

The spindle fibres disappear, the nuclear membrane & nucleolus reappears in

each nucleus.

Chromosomes uncoil

Cytoplasm dividesinto two cells

Cytokinesis occurs

No interphase, no replication of chromosomes

Meiosis: Prophase II

Chromosomes become visible

Spindle forms

If nuclear membrane reformedafter Telophase I, it will break downnow
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Metaphase II

Anaphase II

Telophase II

Chromosomes lined up at the equator

of the cell facing opposite poles

Each sister chromatid is attached tothe spindle fibres at the centromere

The sister chromatids together split

Chromatids separate

Spindle fibres pull each chromatid

to opposite poles

Centromere are divided

Chromatids reach the poles &

become new chromosomes

Nuclear membrane & nucleolus

form again at each chromosome

Chromosomes become extended

& not visible

Cytokinesis occurs & 4 haploid

daughtercell are formed.


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THE COMPARISON BETWEEN MEIOSIS I & MEIOSIS II

MEIOSIS I SIMILARITIES MEIOSIS II

Consist of 4 stages : P,M,A,T

Involve division of nucleus & cytokinesis

DIFFERENCES

Occur Synapsis Not occur

Yes Cross over No

MI-paired

homologous

chromosomes line

up at the equator

Metaphase MII each

chromosome with

sister chromatids

line up at the

equator

AI pairedhomologous

chromosomes

separate & move

to opposite poles

Anaphase AII the sisterchromatids separate

& move to opposite

poles

2 haploid cells are

formed

At the end 4 haploid cells are

formed


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THE COMPARISON BETWEEN MITOSIS & MEIOSIS

MEIOSIS SIMILARITIES MITOSIS

Division of cells

The chromosomes replicates only once

DIFFERENCES

In reproductive organ Place occ ur In somatic cell

Parent cell divides twice Number of d iv is ions Parent cell divides once

Four haploid daughter cells Number of daugh ter

cel ls

Two diploid daughter cells

Occurs during prophase I Synapsis of

homo logous

chromosomes

Not occurs

Occurs twice Number of c ytok ines is Occurs once

Occurs during prophase I Cross ing over of

chrom at ids of

homo logous

chromosomes

Not occurs

Half of number of

chromosomes of the parentcell (haploid)

Number of

ch romosom es indaugh ter cel ls

Genetically identical to the parent

cell

Genetically non-identical to

the parent cell & each other

Genet ic comp osi t ion of

daugh ter cel ls

Genetically identical to the parent

cell

Occurs once during

interphase before meiosis I

DNA repl icat ion Occurs during interphase before

mitosis begins


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Chapter 5 Bio f4